Displaying video content that was originally created for television on a computer display would be a useful feature of a multi-media computer system. However, television video signals and the video formats of most computer displays are not the same. Computers commonly use a non-interlaced video display format, also called a progressive scan. That is, an entire frame is scanned line-by-line, typically from top to bottom. The process repeats and re-scans the entire frame at a given refresh rate. In contrast, many sources of consumer video, such as television signals, use an interlaced display format.
Interlaced systems interleave two or more fields to display an entire frame. A typical U.S. NTSC television signal uses two fields, each with video data for every other horizontal line of the entire frame, and consecutively scans the two fields 30 times per second. The viewer perceives the resulting image as a blend of the two individual fields with an effective refresh rate of 60 Hz. If instead, the entire frame was scanned progressively at 30 Hz, the viewer might perceive a flicker of the displayed image. The flicker is much less apparent with higher refresh rates such as 60 Hz. The interlaced format thus reduces flicker without requiring a doubling of the data transfer rate needed to update the entire frame at 60 Hz. Other interlaced video formats are also common, such as the 50 Hz phase alternation line (PAL) system used in Europe. All such interlaced formats use the technique of interleaving fields to create an entire frame, and the present invention is not intended to be limited to deinterlacing any particular interlaced video format.
Modern computer systems are not constrained by data transfer rates required for the noninterlaced format to the same extent as were early television systems. However, the use of interlaced display formats for many consumer video products is still common. In order to display this interlaced material on a non-interlaced computer display there is a need for deinterlacing.
A conventional deinterlacing method applies a vertical filter to scale up a field to construct a frame for display. This method is called a line doubler in pro-scan television set or bob method in the PC graphics industry as different vertical offsets of the odd and even fields are adjusted as part of the up scaling. Due to the loss of vertical resolution within one interlaced field, flickering artifacts can be observed. This is more pronounced in relatively static areas that contain high vertical frequency details.